This invention relates to an electrode system for nickel-cadmium (nicad) batteries and to a procedure for its manufacture. The invention applies in particular o electrode systems for nicad batteries in which at least one type of electrode is produced by employing fiber-structure technology.
Conventional nicad batteries are produced with so-called self-baking or sintered electrodes. Sintered electrodes can be manufactured only up to a particular thickness, thus limiting the available energy density. The development of fiber-structure technology has brought improvements in this area. It employs metallized fiber structures which are produced by metallizing porus, nonwoven or needle-bonded fabric of electrically nonconducive, synthetic fibers. Fiber electrodes can be made thicker, thus allowing for smaller battery dimensions with the same energy density or the same battery dimensions with more power. As another advantage, fiber electrodes last longer. However, they do require a somewhat higher charging voltage than cells with sintered positive electrodes. This means that the need for higher charging voltages must be taken into account when using nicad batteries with fiber electrodes. Higher charging voltages in turn can have drawbacks in terms of water consumption.
Alkaline cells, like nicad batteries, can be of the sealed or open type.
In an open design, the gases forming at the charge or feed end are dissipated. The separator not only shields the electrodes but also prevents the gases from penetrating.
By contrast, in a sealed battery design the oxygen generated at the positive electrode is channeled directly to the split negative electrodes. This reduces the charging voltage at the feed end. In gas-tight nicad batteries with fiber electrodes the oxygen travels via the gas phase and a porous, gas-permeable intermediate layer between the negative electrodes. The sealed design makes it necessary to limit the amount of electrolyte so as to maintain adequate electrolyte-free gas passages. In the manufacture of sealed, fiber-technology-based nicad cells, the gas is evacuated from the cell enclosure and the unit is permanently sealed so as to control and facilitate the gas flow. Limiting the amount of electrolyte substantially reduces the thermal absorptivity of the cells, thus increasing the risk of overheating in the event the cells are overcharged.
Given the state of prior art as described, it is the objective of the present invention to provide the ability to produce prismatic nickel-cadmium batteries unlimited relative to the amount of electrolyte and incorporating fiber-structure electrodes which, while maintaining the charge-voltage-related advantages over conventional nicad batteries, are compatible with the latter, i.e. they do not require higher charging voltages and in terms of water consumption they need less maintenance. As an added objective, they should be easier to manufacture.
The proposed procedural approach to attaining that objective involves a process for the manufacture of electrode systems for nicad batteries involving at least the partial use of fiber electrodes, whereby positive and negative, lamellar electrodes are produced and, with separator material interpositioned, stacked in alternating fashion in a defined number to form an electrode assembly, with the respective equidirectional electrodes mutually connected by means of connecting straps; the said electrode assembly is pressed into a unitized, coplanar, solid block under compression of the separator material interlayered between the electrodes in a manner that at least in the pressed and installed state the unit impedes essentially any gas transfer in a direction parallel to the surfaces of the lamellar electrodes while permitting such gas transfer in a transverse direction relative to the lamellar electrodes, with cavities being provided for the temporary storage of gas.
The material employed for the separator layers is a nonwoven fiber fleece at least 0.5 to about 1 mm thick which offers sufficient gas storage capacity and corresponding weight per unit area, which can be entirely or partially covered with electrolyte and which preferably consists of a polyamide or polyolefin or a similar polymer or a mixture of these substances, with polyamide being the preferred material. The separator material should maintain a lasting degree of elasticity assuring permanent, mechanically flush contact with the electrodes.
Surprisingly, it has been found that the electrode assemblies produced by the method according to this invention not only do not alter the advantages of fiber electrode-based nicad batteries over conventional nicad batteries but in fact need only low charging voltages, making them compatible with the other systems, while at the same time requiring less maintenance due to substantially less water consumption compared to conventional nicad batteries. This is due to the retention of the oxygen formed in the transfer phase in the electrode assembly and its subsequent dissipation at the negative electrode, as well as to improved thermal balance which is further helped by the application of lower charging voltages. The key requirement is that the compression take place prior to the so-called start-up charge, meaning the initial charge, and that the electrode assembly be locked in place in its compressed, undeformable state.
The assumption is that the oxygen absorptivity and retention capacity of the separator material and the consequently uniform oxygen dissipation at the negative electrode constitute an essential aspect of this invention. From among the many different separator materials, only fleece-type separators lend themselves to this purpose. They consist of statistically distributed, i.e. random fibers in contrast to directionally structured fabrics containing fiber combinations following specific preferential directional patterns. Unlike other separator materials, these nonwoven fiber fleece separators are highly porous, their porosity factor being between 70 and 90%. Due to the elasticity of the fleece separators relative to their thickness perpendicular to the electrode surfaces, the compression and positional fixation during the stacking of the cells will cause the separators to completely fill the space between the electrodes. With a moderate, controlled charge rate, i.e. charging current, the gaseous oxygen produced at the positive electrode will not escape from the electrode assembly but will fill the pores, displacing the electrolyte. At the negative-electrode interface the oxygen is dissipated, depolarizing the electrode. A controlled charge rate can be obtained for instance by keeping the charging voltage constant, a type of control that is not suitable for the aforementioned gas-tight nickel-cadmium cells. The oxygen generated at the positive electrodes during operation is neither removed from the space between neighboring electrodes nor directly channeled to the negative electrodes by the separator material; instead, it is only partly fed to the negative electrode and partly stored in interim fashion. This assures an extended, continuous gas feed to the negative electrode, limiting the charge state of the latter and thus the polarization that would engender a rising, final charging voltage. The even oxygen dissipation and the surplus electrolyte reduce the chance of an overheating. Compressing the electrode assembly blocks the separator to any oxygen movement parallel to the electrode surfaces while in the transverse direction a resistance to the oxygen transfer is produced that causes the oxygen to be stored. It is important that a significant portion of the oxygen that formed in the transfer phase, perhaps between 70 and 98%, be dissipated in that fashion, even though the cell would function without quantitative limitation of the electrolyte. In fact, the cells contain a substantial electrolyte surplus, with the level of electrolyte extending to above the upper edge of the stacked-electrode unit in a manner similar to that in open nickel-cadmium cells.
It is desirable to obtain compression to a predefined inter-electrode spacing and/or a specified state of compression of the separator and/or with a specific compression-force setpoint.
According to one desirable approach proposed by this invention, the separator selected consists of a permanently elastic material which, interlayered between the electrodes, is preferably compressed to 70 to 90% of its original thickness. The separator material used is in the form of so-called fiber fleece which, according to a particularly advantageous concept within this invention, is applied in a multilayer configuration. In another desirable embodiment of this invention the separator can be in the form of pockets surrounding the electrodes. This will automatically produce a multilayer configuration which also serves to protect the edges. Moreover, the pocket approach significantly facilitates the handling of the electrodes.
Apart from the above, the separator performs its usual, conventional functions, providing electrical isolation and offering at least traditional ion conductivity with a full level of electrolyte.
As a desirable aspect it is proposed that prior to the placement of the separator material the electrode plates be surface-leveled, referred to as a so-called precalibration. According to one concept of the invention, such precalibration can be performed on filled and dried electrodes by mechanical means using rollers, whereby protruding or dendrite-promoting metallized fiber tips or metallized residual fibers are largely removed, minimizing the roughness of the electrode surfaces. Minimized surface roughness is particularly important, given that the compression of the electrode assembly produces a permanently elastic, full-face contact between the surface of the lamellar electrode and the separator material which virtually eliminates any gas migration along the surface of the electrode plate.
The compression and/or fixation of the electrode assembly can be performed prior to installation in a cell enclosure or even in a cell enclosure.
The application of the procedure according to this invention has been found to be particularly advantageous for electrodes employing metallized fiber structures produced by metalizing porous nonwoven fleece materials or needle-bonded fabric consisting of electrically nonconductive synthetic fibers, where the textile base structure displays a porosity of between 50% and 98%, the fiber diameter is between 5 and 40 m, the weight per area unit of the nonwoven fleece or needle-bonded fabric is between 50 g/m and 800 g/m and the thickness of the textile material is in the range between 0.5 and 5 mm.
With the procedure according to this Invention it is possible to produce nicad batteries which are smaller and lighter than conventional batteries of equal capacity, which are compatible in terms of the charging voltage and which require a comparable measure of maintenance. The electrode systems manufactured employing the method according to this invention are novel and neither known nor suggested on the basis of prior art.
In terms of the devices employed to attain the objective, the invention proposes an electrode system for nicad batteries at least partially equipped with fiber-structure electrodes, whereby a predefined number of positive and negative electrodes produced in lamellar form are stacked in alternating fashion with the interlayering of separator material to constitute an electrode assembly in which the equidirectional electrodes are mutually connected by means of connecting straps; the said electrode assembly is pressed, between mechanical clamps, into a unitized, coplanar, solid block under compression of the separator material interlayered between the electrodes. It is desirable to use a separator material which at least in the compressed and installed state impedes essentially any gas transfer in a direction parallel to the surfaces of the lamellar electrodes while permitting such gas transfer in a transverse direction relative to the lamellar electrodes, with cavities being provided for the intermediate storage of gas.
The material employed for the separator layers is a nonwoven fiber fleece sufficiently thick to ensure adequate gas storage capacity and corresponding weight per unit area, which can be entirely or partially covered with electrolyte and which preferably consists of a polyamide or polyolefin or a similar polymer or a mixture of these substances, with polyamide being the preferred material. The separator material should maintain a lasting degree of elasticity assuring permanent, mechanically flush contact with the electrodes.
Prior to installation in a cell enclosure, the electrode plates can first be pressed and then locked in place by appropriate retaining elements. For example, frame-clamping plates can be placed on the outer electrode plates and, after the compression process, connected by means of clamping bars extending along the lateral edges of the electrode plate. The frame can be welded on, attached with screws or the like and/or made adjustable, allowing for the adjustment of the thickness and pressure values of the assembly.
As a desirable aspect it is proposed that the outer electrode plates be negative plates on which the clamping frame can be mounted without requiring the interpositioning of separator material.
The electrode system according to this invention permits the manufacture of advanced nicad batteries which compared to conventional nicad batteries of equal capacity are lighter and smaller, which are compatible in terms of the necessary charging voltage and require a comparable measure of maintenance. The electrode systems according to this invention are novel and are not known or suggested on the basis of prior art.